e are widely regarded as a nation of couch potatoes but, according to the Fitness Industry Association, that could be about to change. It is predicted that there will be a 10 per cent growth in the fitness industry this year, and the major operators have earmarked around pounds 750m for health and fitness projects over the next three years. Soon we won't be able to move for Stairmasters and ab crunchers.

After the annual January rush of get-fit resolvers has calmed, around 6 per cent of us regularly work out at a gym. The reasons are more complicated than first appears. People join gyms because they want power over their own lives: physical supremacy coupled with mental focus. But this impulse is often rapidly overtaken by another compulsion, for the endorphin fix that exercise gives. "I've continued working out because it's become part of my way of life," says Eddie Scott-Bennin, who goes to the gym for one hour five days a week. "I need the rush that I only get from training."

Patsy Alcide, 31, visits her gym for two-and-a-half hours every day. "I think I've done everything I can do to my body," she says. "I'm quite happy with it now but I want to maintain it. It's become an addiction. If I don't come I feel sick; the gym releases my stress. On the machines my mind is clear. It's my way of releasing everything."

Every session at the gym has its own integral routine: warm-up, cardiovascular, weights, stretches, a series of actions that the body can move through without thought, treading the same steps every day, pushing the same weights. Your body and brain learn when to switch off and switch on.

The first few months are a struggle, in which slack muscles fight against unnatural repetitive movements. Ann Able, 46, has been going to a gym since last February and has lost about 20kg. "Some of the kids here are so fit that it must be hard to keep going and feel they're improving; I know I'll keep improving. The cardiovascular work is a bit boring but now I just listen to my Walkman and go off into my own world." The exertion slowly transforms into exhilaration when you discover that you can do it. Instead of being a painful alien element, the gym becomes part of your life - "Otherwise," as Able says, "I'd be at home feeding my face and watching TV." Where once your body was battling to fit in with the beat of the machines, it can now work with them and a new rhythm takes over; you challenge the machines to go faster, heavier and harder. Suddenly you are in control and you have the power to bend equipment to your needs.

Whether you started working out to get fit, lose weight or develop your body, inevitably the day will come when that initial goal is achieved and goes past unnoticed - you could stop and leave now, but you don't. What keeps you going is not simply the routine and rhythm but the experience of the gym itself: the fix of adrenaline, the sense of physical purity and that high moment of controlled strain, akin to orgasm or defecation, that comes every time you push the body just a little bit harder, just a little bit faster.

Danny Green, 34, works out for around 10 hours a week. "Eight times out of 10, you come here and you feel good, but then sometimes your mind clicks off and it won't go. There are a lot of mind games involved and sometimes you're fighting a losing battle. But if your mind's on it, it's one of the best feelings: achievement. That's when you really get the buzz. If you no longer feel the pain it's no good. I stopped coming for six weeks and it tore my heart out. I got stressed, migraines; a big part of my life just wasn't there."

Of course there are those who work out to keep themselves in shape for football or some other sport, but for most, the gym is an end in itself, the ultimate self-seeking form of exercise - exertion without purpose. There is none of the companionship of team sports, none of the mental invigoration of squash, none of the elemental luxury of swimming. The pursuit of all this perfect, sculpted, functionless flesh seems a terrible waste, as if Kasparov had dedicated his life to playing electronic chess. Watching all these beautiful bodies laid out in pumping, uniform rows like the human equivalent of battery chickens, it seems extraordinary that gyms have developed such a reputation as pick-up joints. The only thing that the owner of the true gym body is focusing on is him or herself. If you are eyeing up someone else, then something is wrong - why else would they be surrounded by so many mirrors?

The kind of panic that seems to be catching up with us at the end of the century could go some way to explaining the growing membership of gyms. We are more stressed, more anonymous, more helpless in the face of technology and information than we have ever been before. Maybe we are trying to balance the loss of control over our lives with an area in which we have absolute power. By working out, we are taming our bodies and taming our minds. Control at the gym gives us control in the rest of our life.

To the unworked-out, the lure of the gym seems to be based purely on physical self-improvement - a kind of religious dedication to appearance. In reality that small, neon-lit room filled with loud music and human sweat is an impossible model for a new society. There is a kind of democracy where everyone, old or young, strong or weak, uses the same machines and does the same exercises.

"My non-gym friends think it's crazy," says 21-year-old Sharon Ryan, who clocks up 22 hours-plus per week, "but they still ask me for advice. I don't mean to look at them critically but their bodies have changed, they've put on weight and stuff. Coming here's a buzz, it takes you mind off things; time flies. It's like you're back at school doing PE; everyone's on the same level, irrespective of colour or race or age, we all treat each other the same. I work long hours, but as soon as I finish I come here. The gym is my social life"

These photographs, taken at a variety of gyms in and around London, will be exhibited in `Signs' at the Atlantis Gallery, 146 Brick Lane, London E1, 8-21 February. All interviews conducted at the Portobello Green Fitness Centre, London.